Digital servo control method for controlling a head driver for moving a head to a target track

ABSTRACT

The present invention relates to a servo control apparatus of a disk recording system and particularly to a servo control apparatus and method which are capable of controlling position of a head for recording digital data on a disk as recording media and reading the digital data. A digital servo control method for controlling a head driver for moving a head for reading servo information to a target track every a predetermined sampling period in a data storage system using disk recording media including the servo information indicating a divisional position of a recording area comprises the steps of detecting a current track position of the head in a current sampling period by determining information on a head position on a disk from the servo information, determining the current track position, an estimated speed in the next sampling period for moving to the target track by a control signal inputted in the head driver so that the head can be moved to the current track position, and an estimated position track of the head in the next sampling period, calculating a track interval between the target track and the estimated position track, and a target speed to move the head in correspondence with the interval, and calculating a difference value between the target speed and the estimated speed to thereby output a control signal corresponding to the difference value to the head driver.

This is a division of application Ser. No.08/343,939, filed Nov. 17,1994.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to, incorporates herein and claims allbenefits occurring under 35 U.S.C. §119 from our application earlierfiled in the Korean Industrial Property Office on Nov. 27, 1993 of ourapplication entitled DIGITAL SERVO CONTROL APPARATUS AND OD OF DATASTORAGE SYSTEM USING DISK RECORDING MEDIA, which application was dulyassigned Serial No.: 25500/1993.

BACKGROUND OF THE INVENTION

The present invention relates to a servo control apparatus of a diskrecording system and particularly to a servo control apparatus andprocess capable of controlling the position of a head for recording andreading digital data on a disk as recording media.

In a current disk recording system, a hard disk drive (hereinafterreferred to as HDD) operates in two modes, in accordance with the movingdistance, the first of which is called a seek mode, in which a headmoves between tracks so as to reach target position, and the second ofwhich is called a track following mode, in which the head is accuratelypositioned on a data line of the track after the head has reached thetarget track.

In a method for controlling the position of the head in a hard diskdrive, the control of velocity is executed until the head reaches atarget position and the control of position is executed at the targetposition, so that the head lies on the track.

In a typical hard disk drive, the seek mode executes a search mode inthe first deceleration phase and executes a transition mode in thesecond deceleration phase. Therefore, the seek mode is separated intothe search mode and transition mode, in which servo information havingthe course of feedback uses a track number with a gray code. In thetrack following mode however, servo information uses a position errorsignal (hereinafter referred to as PES) using A and B bursts.

In such a conventional control method for respective seek (and itssearch and transition modes) and track following modes, a compensator ora proportional integral derivative controller has been frequently used.A voice coil motor controls the position of head in the disk recordingsystem.

The conventional servo control apparatus may be an analog controlapparatus depending upon the hardware to be controlled, or a mixedanalog and digital mixing control apparatus. First, the head reads thenumber of the track in which the head is positioned currently throughthe gray code value indicating the servo information of track for eachsampling period. As a result, the moving distance by which the headshould be moved from a current position to a target position is defined.Target velocity corresponding to the moving distance is obtained from alook-up table. When the moving distance is defined, the seek mode isalso determined. A real velocity is, however, determined by thedifference between the value prior to sampling and current samplingvalue of the current position. Therefore, in the seek mode beingseparated as the search mode and transition mode, the control ofvelocity is executed by a control input derived from the target velocityand real velocity.

The conventional servo control apparatus as discussed above however, hasthe following problems which should be cleared. First, the real velocityis calculated for each sampling interval. The value calculated tends tobe come more inaccurate however, where the sampling interval is shortand the real position has changed substantially. Thus, there is aproblem when the voice coil motor is required to execute an unexpectedor abnormal movement in the search mode the head during the search modeor the transition mode.

Secondly, the track following mode which has a full state feedback whichuses position information (a number of track, PES) and velocityinformation containing expectable errors, has great difficulties inmaintaining the complete control due to instability of the real velocityinformation.

Third, it is possible that a steady state error will occur during thetrack following mode because there is no method for directly removing anexternal disturbance. While such an error is capable of being indirectlycontrolled by using an integral controller, but is not capable of beingcontrolled over the whole range of data on the recording media, so thatthe error is not completely removed.

U.S. Pat. No. 5,182,684, to Thomas et al. for an Estimator PositioningSystem And Method, endeavors to control the movement of a head of a diskfile by generating a position signal in response to servo informationread. The current position signal is sampled regularly, and togetherwith a current actuator control signal, updates estimated velocity. Thedistance from the current position to a specified position, ie.,distance-to-go, is also updated regularly. When the distance-to-go isgreater than seven tracks the control signal is generated according to amaximum velocity of the actuator. When a maximum velocity is reached thesystem switches to a coast mode or when the number of tracks is lessthan or equal to seven tracks the system switches to a decelerationphase. During the deceleration phase the actuator control signal isgenerated by squaring the estimated velocity and dividing the result bythe distance-to-go. Thomas et al. also suggests using an estimatedposition instead of the actual current position signal. In practice,only the position information is calculated and is used to modify thedistance-to-go at each sampling period. The deceleration phase isfollowed by a track-following mode when the distance-to-go is between1.0 and 0.125 tracks, i.e, less than 0.25 tracks.

U.S. Pat. No. 5,164,931, to Yamaguchi et al. for a Method And ApparatusFor Control Of Positioning, uses a system for controlling positioning ofa recording/playback head. The head is initially controlled as avelocity control system wherein a target velocity is generated inresponse to a track number signal. The difference between the headvelocity and the target velocity is determined and the difference issupplied to a power amplifier for controlling the actuator for theheads. When the head reaches the vicinity of a target position, the headis controlled as a position control system utilizing the values of thetarget position and the position error signal of the current position ofthe heads.

U.S. Pat. No. 5,126,897, to Ogawa et al. for a MagneticRecording/Reproducing Apparatus Capable Of Reducing A Setting Time In AHead Positioning Control Mode,for a servo type magnetic head drivecontrol device with the moving velocity of the head controlled accordingto the moving distance to a target position in response to servo dataand target velocity data. The servo data is used to generated positioninformation and velocity information. Using the position information andtarget position information for calculating a moving distance, targetvelocity information is generated, in accordance with the movingdistance. The velocity of the head is controlled for moving the head toa predetermined position on the basis of the difference between thevelocity information and the target velocity information. After acertain time period or when a predetermined position is reached, controlof the head movement is changed to be dependent upon a centralprocessing unit (CPU) for operating in a positioning control mode havingreduced settling time. In the position control mode, the CPU detects theposition of the head actuator/driver using the position error signal(PES) from a head position decoder. The velocity signal, generated inresponse to the position error signal, is also detected by the CPU. TheCPU then uses the position error signal and the velocity signal tocontrol the positioning of the head. Use of a velocity estimated by theCPU in the position control mode is also discussed, by using a controlmodel predefined in the CPU.

U.S. Pat. No. 5,051,851, to Sakurai for a Method And Apparatus ForPositioning Head On The Basis Of Premeasured Amount Of Displacement,describes an apparatus having a plurality of disk-dependent displacementdata stored in a table. The apparatus detects the displacement of a headfrom a designated track of a magnetic disk and controls the positioningof the head in a radial direction in response to disk-dependentdisplacement data from the table according to the designated track andfurther in response to the detected head displacement.

U.S. Pat. No. 5,040,084, to Liu for Disk Drive system And Method,provides servo tracks on a magnetic disk and storing position data ofeach servo track in memory by measuring the servo signal on each track.

U.S. Pat. No. 4,980,876, to Abate et al., for a Single Stage Track SeekMethod, uses a seek method which remains in a positioning modethroughout the seek process. In the seek process the sinusoidal positionerror signal of the head relative to a track center is utilized andcompared to an ideal profile stored in a look-up table. The head ismoved on the basis of the difference between the ideal profile and theactual position.

U.S. Pat. No. 4,954,907, to Takita for a Head Positioning Control MethodAnd System, mentions controlling head movement based on a new measuredhead position, a new measured head velocity, the previously measuredhead velocity, the previously computed actuator control signal, andpredetermined stored constants when generating a new actuator controlsignal.

U.S. Pat. No. 4,920,462, to Couse et al. for a Disc Drive Fine ServoVelocity Control And Method For Head Positioning Relative To A Disc,uses servo velocity control circuitry for positioning a head on adestination track, and servo position control circuitry for centeringthe head in the destination track. The system uses the track identifyingGrey code detected on the disc for moving the head to a track near thedestination track under control of the servo velocity control circuitry,and uses the position error signal calculated in response to thedetected servo data on the disc for centering the head on thedestination track.

U.S. Pat. No. 4,914,644, to Chen et al., a Disk File Digital ServoControl System With Coil Current Modeling, provides long seek mode, ie.,moving a head from track to track, and a short seek mode, i.e., a trackfollowing mode. During initialization a state estimator in amicroprocessor provides four estimated values, ie., estimated headposition, estimated head velocity, estimated voice coil motor currentand windage (a bias force compensation current). Followinginitialization a digitizing PES channel provides primary and quadratureposition error signals to the microprocessor, which in turn computes theactual position of the head and an estimator error is calculated basedon the difference between a predicted head position and the actual headposition. Next, the microprocessor computes a velocity error signal andcompares this velocity error signal to a predetermined threshold. Theestimated head position and estimated head velocity are computed basedon the estimator error, a predicted head position and a predicted headvelocity. The control signal is function of velocity error signal, andthe estimated head acceleration. Following the output of the controlsignal the predicted variables are updated. The control signal isconverted to analog form and provided to an integrated power amplifier.The voice coil motor current output by the integrated power amplifier isfeed back and summed with the analog control signal.

U.S. Pat. No. 4,835,633, to Edel et al. for a Disk File Digital ServoControl System With Compensation For Variation In Actuator AccelerationFactor, stores information about the head velocity, coil current, andPES sampling times, and uses the stored information to calculate, anacceleration factor for generating a digital control signal which isutilized to control movement of the head.

U.S. Pat. No. 4,697,127, to Stich et al., an Adaptive Control TechniqueFor A Dynamic System, controls a voice coil motor in a seek mode andtrack follow mode, wherein the seek mode is performed until the positionof the head is within a quarter track of the target track, at which timethe track following mode is performed. In the seek mode a microprocessoruses position information and an estimated velocity to produce a controlsignal for generating the coil current used in controlling the voicecoil motor. The position error signal and the known coil current areused to generate an estimated bias and the estimated velocity, whereinthe estimated bias is used within the estimator to correct the velocityestimate. In the track follow mode the microprocessor combines position,integrated position and estimated velocity signals to produce acomposite signal which represents the magnitude to be applied to theactuator voice coil.

U.S. Pat. No. 4,679,103, to Workman, for a Digital Servo Control SystemFor A Data Recording Disk File, has a digital servo system whichreceives a digital head position error signal and a digital signalcorresponding to the head actuator input signal, and outputs a digitalcontrol signal. The digital control signal is converted to an analogsignal, integrated and amplified to produce the head actuator inputsignal. The control signal is calculated from estimated head positionvalues, estimated velocity and an estimated actuator input signalrequired to compensate for bias forces. The estimated values arefunctions of respective predicted values.

The present invention has an advantage over the noted prior art discusesabove by using a previous control signal, a plurality of predictionestimates, a table of target values and units of different measure ineach of three modes of operation for achieving stabilized settling ofthe head on the target track.

The transition mode is executed prior to about sixteen tracks from thetarget position, and upon reaching the target position, the advancespeed of the head becomes lower. Although the seeking time becomeslonger, the transition mode is executed for the purpose of stabilizesettling the head. However, the conventional transition mode recognizesthe current position of the head using the gray code and executes thecontrol of velocity with only track position information using the graycode, so that an accurate control can not be executed during theperformance of the transition mode and a bad influence can be effectedon settling the track following mode.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved servo control apparatus.

It is a digital servo control apparatus for differentially controllingthe movement of head in accordance with the changes of operating modes.

It is another object to provide a digital servo control for executingthe servo control for an external disturbance.

It is another object invention to provide a digital servo control methodfor differentially controlling the movement of head in accordance withthe changes of operating modes.

These and other objects may be achieved with one embodiment of thedigital servo control system and method for controlling a head driverfor moving a head for reading servo information to a target track everya predetermined sampling period in a data storage system using diskrecording media including the servo information indicating a divisionalposition of a recording area detects a current track position of thehead in a current sampling period by determining information of a headposition on a disk from the servo information, determines the currenttrack position, an estimated speed in the next sampling period formoving to the target track by a control signal inputted in the headdriver so that the head can be moved to the current track position, andan estimated position track of the head in the next sampling period,calculates a track interval between the target track and the estimatedposition track, and a target speed to moves the head in correspondencewith the interval, and calculates a difference value between the targetspeed and the estimated speed to output a control signal correspondingto the difference value to the head driver.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a graph showing head moving velocity corresponding to a trackmoving distance in a contemporary version of hard disk drive;

FIG. 2 is a graph in detail showing head moving velocity in theacceleration phase of the drive represented in FIG. 1;

FIG. 3 is a block diagram showing a control system of a conventionalservo control apparatus for controlling the position of a head whilereading and writing digital data in a recording system using diskrecording media;

FIG. 4 is a block diagram showing a data storage system using diskrecording media constructed according to the principles of the presentinvention;

FIG. 5 is a block diagram showing a digital servo system according to anembodiment of the present invention;

FIG. 6 is a block diagram showing a construction of an estimatoraccording to an embodiment of the present invention;

FIG. 7 is a block diagram showing a digital servo system during a searchmode and transition mode according to an embodiment of the presentinvention;

FIG. 8 is a block diagram showing a digital servo system during a trackfollowing mode according to an embodiment of the present invention;

FIG. 9 is a schematic view showing tracks on a recording media of a HDDand A and B bursts according to an embodiment of the present invention;

FIG. 10 is a main flow chart showing a control process according to anembodiment of the present invention;

FIG. 11 is a flow chart showing a search mode ISR (interrupt serviceroutine) of FIG. 10;

FIG. 12 is a flow chart showing a transition mode ISR of FIG. 10; and

FIG. 13 is a flow chart showing a track following mode ISR of FIG. 10;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIG. 1 is a graph showing head movingvelocity corresponding to a track moving distance in a general HDD. Thephase from zero to X1 represents an acceleration phase, the phase fromX1 to X2 represents a constant velocity phase, the phase from X2 to X3represents a deceleration phase, and X3 represents a target position.

FIG. 2 is a graph showing in detail showing the head moving velocity inthe deceleration phase from X2 to X3 of FIG. 1, in which the phase fromX2 to X2' represents a first deceleration phase and the phase from X2'to X3 represents a second deceleration phase. The seek mode mentionedabove means an executing mode in the deceleration phase from X2 to X3,as shown in FIG. 1. As shown in FIG. 2, the seek mode executes a searchmode in the first deceleration phase and executes a transition mode inthe second deceleration phase. Therefore, the seek mode is separatedinto the search mode and transition mode, in which servo informationhaving the course of feedback uses a track number using a gray code. Inthe track following mode, servo information uses a position error signal(hereinafter referred to as PES) using A and B bursts.

In the seek and track following respective modes of a conventionalcontrol process, a compensator or a proportional integral derivativecontroller has been usually used.

FIG. 3 is a block diagram showing a control system of a conventionalservo control apparatus for controlling the position of head with regardto reading and writing digital data in a recording system using diskrecording media. Here, the term "plant" represents a voice coil motor(hereinafter referred to as VCM) which controls the position of head inthe disk recording system. The target velocity value in the decelerationphase by the servo system of FIG. 3 is obtained by the followingequation (1):

    V.sub.TBL =K(P.sub.f -P.sub.r).sup.α                 (1)

where V_(TBL) represents target velocity, P_(f) target position, P_(r)current position, and K proportional constant. Proportional constant Kis defined depending upon the acceleration capability of the VCM, and anexponent a is generally used within the range from 0.5 to 1. The targetvelocity value calculated by the equation (1) above is stored in the ROM(not shown) as the velocity value in a look-up table. Explanation forthe conventional servo control apparatus will be described in detailwith reference to FIG. 3.

Depending upon the particular hardware to be controlled, a conventionalservo control apparatus may be an analog control apparatus or a mixedanalog and digital mixing control apparatus. First, the head reads thenumber of the track in which the head is positioned currently throughthe gray code value indicating the servo information of track everysampling period.

As a result, the moving distance X which should be moved from a currentposition P_(r) to a target position P_(f) becomes defined. At this time,the moving distance X is given by the following equation (2):

    X=P.sub.f -P.sub.r                                         (2)

Thereafter, target velocity V_(TBL) corresponding to the moving distanceX is obtained from the look-up table. Further, when the moving distanceX is defined, the seek mode is also determined.

A real velocity V_(r) is, however, determined by the difference betweenthe value prior to sampling and current sampling value of the currentposition P_(r).

Therefore, in the seek mode being separated as the search mode andtransition mode, the control of velocity is executed by a control inputu, from the target velocity V_(TBL) and real velocity V_(r). That is,the control input u is given by the following equation (3) and the inputcurrent is thereafter applied to the VCM;

    u=K[V.sub.TBL -V.sub.r ]                                   (3)

In equation (3), K represents a proportional constant of the controlsystem. Then, when the head reaches the target position P_(f), theexecuting mode is changed to the track following mode.

The track following mode obtains the PES from the A and B bursts andexecutes the position control which renders the head to be settled onthe target track. Here, in case of commonly used PID controller, thecontrol input u is newly determined by the following equation (4):

    u=-K.sub.p P.sub.r -K.sub.v V.sub.r                        (4)

In equation (4), K_(p) represents a position control constant, and K_(v)represents a velocity control constant.

The conventional servo control apparatus as discussed above however, hasthe following problems which should be cleared.

The real velocity Vr is calculated by the following equation (5):##EQU1##

In equation (5), T_(s) represents a sampling interval. The valuecalculated by the equation (5), however, many errors arise in case whereT_(s) is more and more narrow and the real position P_(r) is drasticallychanged. Thus, there is a problem that the VCM lies in an unexpectedabnormal movement in the search mode and transition mode. Further, thetrack following mode using a full state feedback which is constructed asposition information (a number of track, PES) and velocity informationV_(r) having expectable errors, has great difficulties in obtaining thecomplete control performance due to instability of the velocityinformation V_(r). Actually, it is possible that a steady state errorhas occurred in the track following mode because there is no method fordirectly removing an external disturbance. Such an error is capable ofbeing indirectly controlled using an integral controller, but is notcapable of being controlled in the whole range of data on the recordingmedia, so that the error is not clearly removed.

FIG. 4 is a block diagram showing a data storage system using diskrecording media according to an embodiment of the present invention. Inthe construction, the microprocessor 401 is respectively connected to aprogrammable read only memory 403 (hereinafter, PROM) which stores apreset control program and a prediction estimator algorithm of themicroprocessor 401 and to a static random access memory 405(hereinafter, SRAM). A head 407 performs a horizontal movement on a disk409 as recording media and reads/writes data on the disk 409. A VCM 411serves as an actuator and is connected to the head 407 to therebyactivate the head 407 in the horizontal direction on the disk 409. Aspindle motor 413 serves as a rotating actuator, of which the activatingaxis is connected to the disk 409, thus rotating the disk 409. A VCMdriver 415 is connected to the VCM 411 and controls the activationthereof.

A DAC (digital/analog converter) 417 is respectively connected to themicroprocessor 401 and the VCM driver 415. The DAC 417 receives adigital control input signal u from the microprocessor 401 and convertsthe signal into an analog signal to provide an analog-converted signalto the VCM driver 415. A motor driver 419 is respectively connected tothe spindle motor 413 and to the microprocessor 401, and under thecontrol of the microprocessor 401, controls the activation of thespindle motor 413. An amplifier 421 is connected to the head 407,amplifies a signal read by the head 401, and outputs the amplifiedsignal. Additionally, the amplifier 421 amplifies an input signal to bewritten and outputs the amplified input signal to the head 407. Aninterface controller 429 is under the microprocessor 401, and receivesand transmits data together with an external data input device (notshown). A reading decoding and writing encoding unit 423 is connected tothe microprocessor 401, amplifier 421, and interface controller 429,respectively. Under the control of the microprocessor 401, the readingdecoding and writing encoding unit 423 receives writing data from theinterface controller 429 and encodes the data to an analog fluxconversion signal, thereby outputting the signal to the amplifier 421.The microprocessor 401 converts the analog reading signal received fromthe amplifier 421 into a digital reading signal and outputs the digitalsignal as an encoded read data (hereafter, ERD).

An ADC (analog/digital converter) 425 is connected to the readingdecoding and writing encoding unit 423, from which the ADC 425 receivesan analog servo reading signal to convert the analog servo readingsignal to the position error signal PES, thus outputting the PES to themicroprocessor 401. A gate array 427 is connected to the readingdecoding and writing encoding unit 423 and receives the ERD signal, fromwhich the gate array 427 detects and outputs servo information such asgray codes.

In the construction as mentioned above, the microprocessor 401 loads theprediction estimator algorithm and the predetermined control program,received from the PROM 403, into its own interior and executes theoverall control operation in the digital servo control apparatus.

FIG. 5 is a block diagram showing a digital servo system according to anembodiment of the present invention. Here, a prediction estimator in theform of an algorithm is stored into the microprocessor 401. Thisprediction estimator is as a kind of known art, disclosed in "digitalcontrol of dynamic system (2nd edition)", written by G. F. Franklin, etal.

Referring to FIG. 5, K_(DAC) represents a digital/analog converter,K_(ADC) analog/digital converter, K_(amp) power amplifiertransconductance, Km VCM torque constant, T_(m) torque output from VCMactuator, T_(D) exterior disturbance, J swing arm's moment of inertia, a[RAD/S² ] actuator angular velocity, V [RAD/S] actuator angularvelocity, X [RAD] actuator angular displacement, and K_(arm) swing armkinematics. Here, both T_(m) and T_(D) are applied to an adder. FIG. 5functions to convert an angular displacement to a linear displacement.At this time, the linear displacement of the head is predicted as atrack number or the PES.

Here, the track number is indicated as a gray code and means an absoluteposition on the disk. The analog track number recorded in a servopattern on the disk is converted into ERD via the reading decoding andwriting encoding unit 423, and then the ERD is decoded to the gray codevia the gate array 427. The microprocessor 401 receives the gray code asthe track number.

FIG. 6 is a block diagram showing a construction of an estimatoraccording to an embodiment of the present invention.

FIG. 7 is a block diagram showing digital servo control during a searchmode and transition mode according to an embodiment of the presentinvention.

FIG. 8 is a block diagram showing digital servo control during a trackfollowing mode according to an embodiment of the present invention.

Referring to FIGS. 6 to 8, P_(est) represents estimated positioninformation calculated by the estimator, V_(est) estimated velocityinformation calculated by the estimator, W_(est) estimated bias forcecalculated by the estimator, V_(TBL) target velocity value in a velocitycurve (in the look up table), K controller gain, L estimator gain, Xreal state variable vector, X estimation state variable vector, y outputvector of plant, y estimation output vector of model, and T_(D) externaldisturbance.

FIG. 9 is a schematic view of a servo frame showing tracks on recordingmedia of an HDD and A and B bursts according to an embodiment of thepresent invention.

FIG. 10 is a main flow chart showing a control process according to anembodiment of the present invention.

FIG. 11 is a flow chart showing a search mode ISR (interrupt serviceroutine) of FIG. 10.

FIG. 12 is a flow chart showing a transition mode ISR of FIG. 10.

FIG. 13 is a flow chart showing a track following mode ISR of FIG. 10.

Explanation on a digital servo control apparatus and method in datastorage system using disk recording media according to an embodiment ofthe present invention will be in detail described hereinafter withreference to FIGS. 5 to 13.

Firstly, of the digital servo control system in data storage systemusing the disk recording media, a model of a plant can be given as akind of state equation by the following equation (6):

    X(k+1)=ΦX(k)+Γu(k)                               (6)

In this case, the φ and Γ represent matrixes for indicating plant gains,X(k) represents real state variable vector as indicated by the followingequation (6-1): ##EQU2##

Here, P_(r) represents a real position, and V_(r) represents a realvelocity.

In the preferred embodiment of the present invention, to reduce theestimation error of the real velocity V_(r), the estimation velocityV_(est) is obtained by using a prediction estimator. As a result, theprediction estimator shown in FIG. 6 can be obtained by the followingequations (7) and (8):

    X(k+1)-φX(k)+Γu(k)+L[y(k)-y(k)]                  (7) ##EQU3##

In this case, P_(est) represents the estimated value of the realposition P_(r), and V_(est) represents the estimated value of the realvelocity V_(r) In the equations (7) and (8), the output vector [y(k),y(k)] of the plant and model becomes [P_(r) (k), P_(est) (k)]. Lrepresents a feedback gain matrix of the prediction estimator.

It is understood that information on the estimated position P_(est) andon the estimated velocity V_(est) in FIG. 6 are obtained through theestimator, with information of the real position P_(r) estimated fromthe plant.

In addition, in the preferred embodiment of the present invention, todirectly remove the external disturbance, a bias estimation is used.Assuming that the external disturbance W is only a given constant, incase of applying a bias model, the state equation is given by theequations (9) and (10):

    X(k+1)-φ.sub.ω X(k)+Γ.sub.ω u(k)     (9) ##EQU4## here,

In the equation (10), W_(est) represents estimated bias value. φ and Γof the equations (6) and (7) do not correspond to φ₁₀₇ and Γ.sub.ω ofthe equation (9), since construction of the estimation state variablevector X(k) are different from each other as indicated in the equations(8) and (10). The bias estimation as the equation (9) is applied only inthe track following mode.

Further, in the preferred embodiment of the present invention, both thenumber of track by a conventional gray code and position error signalPES using A and B bursts are applied for the purpose of raising controlresolution of the transition mode. This is indicated by the followingTable <1> showing the position information corresponding to the seekmode. Also, the Table <1> illustrates state of the control systemaccording to control modes of FIGS. 7 and 8.

                  TABLE 1                                                         ______________________________________                                               Mode                                                                                                     Track                                       Class     Search       Transition   Following                                 ______________________________________                                        Control state                                                                          Velocity control                                                                           Velocity control                                                                          Position control                            Deceleration                                                                              First               Second                                                                               Non-existence                          Velocity Curve                                                                           Deceleration Phase                                                                        Deceleration                                                                  Phase                                                  Estimation  State  Variable  X                                                          ##STR1##                                                                                   ##STR2##                                                                                  ##STR3##                                   Measurement                                                                               Gray          Gray Code                                                                                 PES (A and B                            Position       Code        and                  Burst)                        Information                                                                                                  PES                                            Control    u = K.sub.i [V.sub.TBL - V.sub.cst ]                                                      Identical to                                                                              u = [-K.sub.p P.sub.est -                  Equation                      Search Mode                                                                            K.sub.v V.sub.est - W.sub.est ]        Prediction                                                                             Search and Transistion Mode                                          Estimator                                                                                 X(k + 1) = φX(k) + Γu(k) + L.sub.s [y(k) - y(k)]                             Track Following Mode                                                          X(k + 1) = φ.sub.ω X(k) + Γ.sub..omeg             a. u(k) + L.sub.T [y(k) - y(k)]                                      ______________________________________                                    

In the Table <1>, the gain which should be determined in designing aservo control, becomes the controller gain K and estimator gain L.Accordingly, the gains are arranged in accordance with the search andtransition modes and the track following mode, as indicated by thefollowing equations (11) to (14).

Search and Transition Mode

    K=K.sub.1                                                  (11) ##EQU5## Track Following Mode

    K=[K.sub.p K.sub.v ]                                       (13) ##EQU6##

In the equations (11) to (14), K and L matrix values are determined bythe application of a known pole-placement method, which method considersa damping coefficient and settling time from a secondary model.

According to the preferred embodiment of the present invention, so as toimprove the control capability of the transition mode, good velocitycontrol is achieved not by using the track position information by theconventional gray code, but is achieved by both the track positioninformation and position error signal PES.

The method for calculating the current track position P_(r) inaccordance with each mode, is given by the following Table <2>.

                  TABLE <2>                                                       ______________________________________                                                                          Measurement                                 Mode                 Resolution per Track                                                                        Variable                                   ______________________________________                                        Search    Track     1             Gray Code                                   Transition                                                                                      Tran-Unit                                                                                  16            Gray Code                                                                           and PES                    Track Following                                                                               PES               1024                                                                                       PES                            ______________________________________                                    

During the search mode, when the moving distance |P_(f) -P_(r) | iswithin sixteen tracks, the search mode is changed to the transitionmode. Thereafter, when the moving distance |P_(f) -P_(r) | reaches therange of the target position P_(f), that is of eight tran-units, thetransition mode is changed to the track following mode.

FIG. 9 is a schematic view of a servo frame showing tracks in an HDD andA and B bursts according to an embodiment of the present invention. Anexample of calculation of the tran-unit in the transition mode will begiven hereinafter with reference to FIG. 9.

In the transition mode, one track is calculated at sixteen tran-units,and sixteen control resolution exists within one track by the PES.Firstly, it is assumed that the current position head has escaped thecenter of `N` track to thereby be positioned on a point `C`, if thetarget position indicates `N+3` track, the real moving distance of thehead corresponds to three tracks, but in view of the tran-unit, the realmoving distance of head corresponds to (three tracks x sixteentran-units)+α. Here, the position at the point `C` of the `N` track isdetermined by the PES.

In the track following mode, one track is calculated at +512 to -511 PES(1024 resolution). Accordingly, tran-unit within one track is calculatedfrom dividing the PES into 64. If the position error signal has 256resolution in the point `C` where the head is positioned, α correspondsto 4 which is obtained by dividing 256 by 64. As a result, in thetransition mode of FIG. 9, the head position to be moved becomes 52tran-units by adding 4 to 48.

The velocity curve of the transition mode (in the second decelerationphase of FIG. 2) is comprised of the moving distance corresponding to256 resolution which is equal to 16 tracks times 16 tran-units. In FIG.9, 52 tran-units corresponds to 3.25 tracks, therefore 3.25 tracks times16 tran-units results in a velocity corresponding to 52 moving distancesand is defined as the target velocity, thereby executing a velocitycontrol operation.

The look-up table is comprised of the deceleration profile in the searchmode and transition mode, and of K_(d) (forward system gain), K(controller gain), and L (estimator gain) requisite for the search,transition, and track following modes.

The deceleration profile during the search mode is applied from 250tracks prior to the is target track, and the deceleration profile duringthe transition mode is applied from 16 tracks prior to the target track.Each deceleration profile is given by adjusting the exponent α of thefollowing equation (15):

    V=K·X.sup.α                                 (15)

In the equation (15), V represents target velocity, X target movingdistance, and K proportional constant. The K is defined depending uponthe acceleration capability of the VCM. In the preferred embodiment ofthe present invention, a value in the search mode is 0.8, and α value inthe transition mode is 0.85, respectively. Of course, the unit of thetarget moving distance X corresponds to the track in the search mode,and to the tran-unit in the transition mode.

The gains discussed above are in the different state, since the overalloperating range of head is divided into six operating points. Theforward system gain K_(d) would be given by the equation (16): ##EQU7##

In the equation (16), the gain K_(d) is shown in a continuous system.When real simulation operates, the gain corresponding to a digitalsystem considering sampling time should be used. Further, in theequation (16), most changeable variable according to the operating pointis referred to as K_(m). Thus, to compensate the change of the K_(m)(VCM torque constant), the gains are used at six areas divided in thelook-up table, as discussed earlier.

An embodiment of the present invention in accordance with each flowchart will be in detail described with reference to FIGS. 4 to 8.

The microprocessor 401 executes an initial operation of a variablerequisite for elements and codes of the VCM driver 415 and gate array427, in step 901. Thereafter, the microprocessor 401 activates the headdisposed in a parking zone when the HDD is stopped, in step 903. In thattime, there is a problem that the head usually sticks to a parking zone,in case of a small-size of HDD, when the head is damaged. In anembodiment of the present invention, so as to solve the problem, thehead has the step of being rocked, which step is well known to thoseskilled in the art, before the step of releasing the lock of head isexecuted. In other words, the head is rocked by repeatedly changing thepolarity of appropriate minute electric current which is applied to theVCM, thereby overcoming the problem that the head sticks to the parkingzone.

In step 905, after the spindle motor reaches a normal velocity (3600RPM), the step of releasing the lock of head is executed for operatingan actuator, which step means, at the early driving of driver, theactuator to which the head 407 is attached is moved from the parkingzone to the position of track zero to prepare the head for a drivingstate.

Thereafter, in step 907, the microprocessor 401 receives the gray codeof track inputted from the head 407 and then senses the current trackP_(r) where the head is positioned. In step 909, the microprocessor 401calculates the interval between the target track P_(f) and the currenttrack P_(r) sensed in the step 907 to thereby calculate the movingdistance X up to the target track P_(f).

The microprocessor 401 executes setting of each mode, for instance, thesearch mode, transition mode, and track following mode, corresponding tothe moving distance X, through steps 911 to 919. Thereafter, in step921, the microprocessor 401 executes an ISR (interrupt service routine)operation in accordance with the modes set in steps above.

In that time, before the ISR operation is executed, the gains of thecontroller and estimator are selected from the table in the mode settingstep, for example, in steps 913, 917 and 919, and a scaling operation ofstate variable is executed, so that the preparation is made for the ISRoperation. A bias calibration operation is performed as an example ofpreparing for the ISR operation. The bias calibration operation is amodule being performed only one time prior to the preparation for thedrive, which module is to compensate the external disturbance occurringin different mechanisms of every drive. In this case, if the differenceof bias force is large, when the actuator runs forward and backward, thecalibration on the bidirection is necessary.

The bias force calculated from the bias calibration is stored in a RAMand is used in the ISR operation. In response to an available memorycapacitance of the RAM and respective characteristics of drive spin-uptime, whether every how many track the bias force is calibrated isdetermined by an initial value in accordance with a referencecharacteristic. According to the preferred embodiment of the presentinvention, when the actuator runs only forward, the calibration of thebias force is executed every 128 tracks.

The ISR operation in step 921 according to the preferred embodiment ofthe present invention will be explained with reference to FIGS. 11 to13.

Each ISR operation of FIG. 10 is performed every sampling time 231.5 μs,which is divided as a search mode ISR operation of FIG. 11, a transitionmode ISR operation of FIG. 12, and a track following mode ISR operationof FIG. 13, respectively.

Referring to the search mode ISR operation of FIG. 11, in step 1101 themicroprocessor 401 outputs a control input value u(k), calculated in theprevious sampling phase (k-1), of the current sampling phase to therebyapply electric current to the VCM 411 through the DAC 417 and VCM driver415. Thereafter, in step 1103 the microprocessor 401 reads current trackidentification through the head 407. In step 1105, the microprocessor401 defines the current position P_(r) from the current trackidentification and in step 1107, calculates estimation control from thecurrent position P_(r). Then, in step 1109 the microprocessor control401 calculates a control input value u(k+1) for the next sampling phase.

If, in step 1111, it is checked that the moving distance |P_(f) -P_(r) |is less than or equal to 16 tracks, the microprocessor 401 registers aservo flag busy state in step 1113 and thereafter, in step 1115,executes the setting for the transition mode. Then the microprocessor401 is returned to the main routine of FIG. 10 to execute theTransistion mode ISR operation. In the step 1111, if it is, however,checked that the moving distance |P_(f) -P_(r) | is over 16 tracks, themicroprocessor 401 executes a counting operation to thereby determinethat a counter time is out in step 1117. Here, if the counter time isout, the microprocessor 401 registers a counter time out error andcompletes the search mode ISR operation. Further, in the step 1117, ifthe counter time is not out, the microprocessor 401 registers the servoflag busy state and continues to execute the search mode ISR operation.

Referring to the transition mode ISR operation of FIG. 12, in step 1201the microprocessor 401 outputs a control input value u(k), calculated inthe previous sampling phase (k-1), of the current sampling phase tothereby apply electric current to the VCM 411 through the DAC 417 andVCM driver 415. Thereafter, in step 1203 the microprocessor 401 readscurrent track identification through the head 407. In step 1205, themicroprocessor 401 defines the current position P_(r), after thetran-unit is determined from the current track identification and instep 1207, calculates estimation control from the current positionP_(r). Then, in step 1209 the microprocessor control 401 calculates acontrol input value u(k+1) for the next sampling phase.

If, in step 1211, it is checked that the moving distance |P_(f) -P_(r) |is less than or equal to 8 tran-units, the microprocessor 401 registersa servo flag busy state in step 1213 and thereafter, in step 1215,executes the setting for the track following mode. Then themicroprocessor 401 is returned to the step 921 of FIG. 10 to execute thetrack following mode ISR operation. In the step 1211, if it is, however,checked that the moving distance |P_(f) -P_(r) | is over 8 tran-units,the microprocessor 401 executes a counting operation to therebydetermine that a counter time is out in step 1217. Here, if the countertime is out, the microprocessor 401 registers a counter time out errorand completes the transition mode ISR operation. Further, in the step1217, if the counter time is not out, the microprocessor 401 registersthe servo flag busy state and continues to execute the transition modeISR operation.

Referring to the track following mode ISR operation of FIG. 13, in step1301 the microprocessor 401 outputs a control input value u(k),calculated in the previous sampling phase (k-1), of the current samplingphase to thereby apply electric current to the VCM 411 through the DAC417 and VCM driver 415. Thereafter, in step 1303 the microprocessor 401reads current track identification through the head 407. In step 1305,the microprocessor 401 defines the current position P_(r) after theposition error signal PES is determined from the current trackidentification and in step 1307, calculates estimation control from thecurrent position P_(r). Then, in step 1309 the microprocessor control401 calculates a control input value u(k+1) for the next sampling phase.

If, in step 1311, it is checked that the moving distance |P_(f) -P_(r) |is less than or equal to 200 PES, the microprocessor 401 executes asettling operation in step 1313 to determine whether the settlingoperation is completed.

In the track following mode according to the preferred embodiment of thepresent invention, the settling operation is determined as 200 PES(which represents an amount of off-track of 20%). On reading and writingoperations, the condition of the settling operation may be differentfrom as mentioned above. That is, on writing operation, if the amount ofoff-track of 20% is maintained during 16 sampling time (3.7 ms), thesettling operation is completed and data is written on the real disk. Onthe other hand, on reading operation, if the amount of off-track of 20%is maintained during 4 sampling time (0.93 ms), the settling operationis completed and data is read from the real disk. Thereafter, if, in thestep 1313, it is determined that the settling operation is completed themicroprocessor 401 terminates the track following mode ISR operation andis returned to the main routine to thereby complete the main routine. Inthe step 1311, if it is, however, checked that the moving distance|P_(f) -P_(r) | is over 200 PES, the microprocessor 401 executes acounting operation to thereby determine that a counter time is out instep 1315. Here, if the counter time is out, the microprocessor 401registers a counter time out error and completes the track followingmode ISR operation. Further, in the step 1315, if the counter time isnot out, the microprocessor 401 registers the servo flag busy state andcontinues to execute the track following mode ISR operation.

In the processes mentioned above, the servo flag is used with 8 bits andindicates the completion of settling operation, error state of the gatearray 427, and the servo error state, on reading and writing operations,which is used in an interface code between the HDD and host.

As discussed above, this invention is advantageous in such ways aspreventing errors occurring in a conventional analog control device oranalog and digital mixing control device by using a pure digital servocontrol apparatus in which the position of head for recording andreading data, is controlled, and making the change of products only withthe correction of a control gain and control step, since this inventionis directed to a pure digital control system using a software.

Further, the present invention is advantageous in such ways as easilycontrolling a voice coil motor by accurately estimating the velocityvariable of head with an estimator, and removing a normal state erroroccurring by an external disturbance by using a bias estimation. Inaddition, the present invention is advantageous in such ways asexecuting an accurate velocity control when compared with a conventionalvelocity control to thereby stabilize a settling operation during atrack following mode, by performing the velocity control with both aposition error signal PES and a conventional gray code in a transitionmode of a seek mode being converted into the track following mode.

What is claimed is:
 1. A digital servo control method for controlling ahead driver for moving a head for reading servo information to a targettrack during every predetermined sampling period in a data storagesystem using disk recording media, with said servo information beingindicative of a divisional position of a recording area, said methodcomprising the steps of:detecting a current track in which said head ispositioned in a current sampling period from said servo information;determining, during said current sampling period, an estimation speedand an estimation position track of said head in the next samplingperiod from a control signal inputted in said head driver and from saidcurrent track; in correspondence with a track interval between saidtarget track and said estimation position track, determining a targetspeed from a look-up table for moving said head to said target track;and outputting a control signal corresponding to a difference valuebetween said target speed and said estimation speed to said head driver.2. The method as claimed in claim 1, wherein said target speeddetermined by:detecting said track interval between said target trackand said estimation position track; comparing said track interval with apredetermined reference interval; determining said target speed incorrespondence with said track interval, when said track interval ismore than said predetermined reference interval; and determining saidtarget speed to move said head in correspondence with a tran-unitinterval by determining said tran-unit interval between said targettrack and said estimation position track from said servo information,when said track interval is less than said predetermined referenceinterval.
 3. A digital servo control method for controlling a headdriver for track-following a head for reading servo information to acenter position of a target track every predetermined sampling period ina data storage system using disk recording media having said servoinformation, said servo information indicating a divisional position ofa recording area, said method comprising the steps of:detecting acurrent position error signal on said target track in which said head ispositioned in a current sampling period from said servo information;determining an estimation speed of said head in the next sampling periodfrom a control signal inputted in said head driver in a previoussampling period and from said current position error signal, determiningan estimation position error signal in the next sampling period, anddetermining an estimation bias force for estimating an external biasforce inputted during the next sampling period; and outputting a controlsignal generated from said estimation speed, said estimation positionerror signal, and said external bias force to said head driver.
 4. Adigital servo control method for controlling a head driver for moving ahead for reading servo information indicating a divisional position of arecording area to a center position of a target track everypredetermined sampling period in a data storage system using diskrecording media having said servo information, said method comprisingthe steps of:(a) detecting a current track in which said head ispositioned in a current sampling period from said servo information; (b)detecting a track interval between said target track and said currenttrack; (c) comparing said track interval with a predetermined referencetrack interval; (d) designating a servo control mode as a first servocontrol mode, when said track interval is more than said predeterminedtrack interval; (e) determining a tran-unit interval by dividing saidtrack interval as a unit of a predetermined tran-unit, when said trackinterval is less than said predetermined reference track interval; (f)comparing said tran-unit interval with a predetermined referencetran-unit interval; (g) designating said servo control mode as a secondservo control mode, when said tran-unit interval is more than saidpredetermined reference tran-unit interval; (h) determining a positionerror signal interval from said current track of said head to saidcenter position of said target track of said head, when said tran-unitinterval is less than said predetermined reference tran-unit interval;(i) comparing said position error signal interval with a predeterminedreference position error signal interval; (j) designating said servocontrol mode as a third servo mode, when said position error signalinterval is more than said predetermined reference position error signalinterval; (k) performing a settling operation when said position errorsignal interval is less than said predetermined reference position errorsignal interval; (l) in correspondence with the designation of saidfirst servo mode, determining an estimation speed of said head and anestimation position track in the next sampling period for moving to saidtarget track from a previous control signal input in said head driver ina previous sampling period and from said current track, outputting acontrol signal for moving said head from said estimation speed, saidestimation position track and said current track to said center positionof said target track to said head driver, and performing said step (c);(m) in correspondence with the designation of said second servo mode,determining a current position in which said head is positioned in theunit of said tran-unit from said servo information, determining anestimation speed of said head and an estimation position in the nextsampling period from said previous control signal and said currentposition, outputting said control signal from said estimation speed,said estimation position and said current position to said head driver,and performing said step (f); and (n) in correspondence with thedesignation of said third servo mode, determining a current position inwhich said head is positioned in the unit of said position error signalfrom said servo information, determining an estimation speed of saidhead, an estimation position in the next sampling period from saidprevious control signal and said current position, and an estimationbias force for estimating an external bias force inputted during thenext sampling period, outputting said control signal from saidestimation speed, said estimation position and said estimation externalbias force to said head driver, and performing said step (i).
 5. Themethod as claimed in claim 4, wherein said step (l) comprises furthersteps of:in correspondence with the designation of said first servomode, determining an estimation speed of said head and an estimationposition track in the next sampling period from a previous controlsignal inputted in said head driver in a previous sampling period andfrom said current track; determining a target speed corresponding tosaid track interval between said target track and said estimationposition track; and outputting said control signal corresponding to adifference value between said target speed and said estimation speed tosaid head driver and performing said step (c).
 6. The method as claimedin claim 4, wherein said step (m) comprises further steps of:incorrespondence with the designation of said second servo mode,determining said current position in which said head is positioned inthe unit of said tran-unit from said servo information; determining saidestimation speed of said head and said estimation position in the nextsampling period from said previous control signal and said currentposition; and determining a target speed corresponding to said tran-unitinterval between said center position of said target track and saidestimation position, outputting said control signal corresponding to adifference value between said target speed and said estimation speed tosaid head driver and performing said step (f).
 7. The method as claimedin claim 5, wherein said step (m) comprises further steps of:incorrespondence with the designation of said second servo mode,determining said current position in which said head is positioned inthe unit of said tran-unit from said servo information; determining saidestimation speed of said head and said estimation position in the nextsampling period from said previous control signal and said currentposition; and determining a target speed corresponding to saidtran-unit, interval between said center position of said target trackand said estimation position, outputting said control signalcorresponding to a difference value between said target speed and saidestimation speed to said head driver and performing said step (f). 8.The method as claimed in claim 4, wherein said reference track intervalis comprised of 16 tracks.
 9. The method as claimed in claim 4, whereinsaid reference tran-unit interval is comprised of 8 tran-units.
 10. Themethod as claimed in claim 4, wherein said reference position errorsignal interval is comprised of 200 position error signal.
 11. A digitalservo control apparatus for controlling a head driver for moving a headfor reading servo information to a target track every predeterminedsampling period in data storage systems using disk recording mediahaving said servo information indicating a divisional position of arecording area, said apparatus comprising:memory means for recording atarget speed according to a predetermined moving distance; detectingmeans for detecting a current track of said head in a current samplingperiod from said servo information; determining means for determining,during said current sampling period, an estimation speed and anestimation position track of said head in the next sampling period formoving to said target track from a previous control signal inputted insaid head driver in a previous sampling period and from said currenttrack; and means for outputting a control signal corresponding to adifference value between a target speed and said estimation speed tosaid head driver by determining said target speed corresponding to atrack interval between said target track and said estimation positiontrack from said memory means.
 12. The apparatus as claimed in claim 11,wherein said determining means is a microprocessor having a predictionestimation algorithm internally stored therein.
 13. A digital servocontrol apparatus for controlling a head driver for track following ahead for reading servo information to a center position of a targettrack every a predetermined sampling period in data storage systemsusing disk recording media having said servo information indicating adivisional position of a recording area, said apparatuscomprising:detecting means for detecting a current position error signalon said target track in which said head is positioned in a currentsampling period from said servo information; determining means fordetermining an estimation speed of said head in the next sampling periodfor moving said head to said center position of said target track by acontrol signal inputted in said head driver in a previous samplingperiod and said current position error signal, an estimation positionerror signal in the next sampling period, and an estimation bias forcefor determining an external bias force inputted in the next samplingperiod; and means for outputting to a control signal for controllingsaid head driver in correspondence with said estimation speed, saidestimation position error signal, said estimation bias force, and saidexternal bias force inputted during said current sampling period. 14.The apparatus as claimed in claim 13, wherein said determining means isa microprocessor having a prediction estimation algorithm internallystored therein.